Root secondary growth: an unexplored component of soil resource acquisition

Gradients in embolism resistance within stems driven by secondary growth in herbs

The stems of some herbaceous species can undergo basal secondary growth, leading to a continuum in the degree of woodiness along the stem. Whether the formation of secondary growth in the stem base results in differences in embolism resistance between the base and the upper portions of stems is unknown. We assessed the embolism resistance of leaves and the basal and upper portions of stems simultaneously within the same individuals of two divergent herbaceous species that undergo secondary growth in the mature stem bases. The species were Solanum lycopersicum (tomato) and Senecio minimus (fireweed). Basal stem in mature plants of both species displayed advanced secondary growth and greater resistance to embolism than the upper stem. This also resulted in significant vulnerability segmentation between the basal stem and the leaves in both species. Greater embolism resistance in the woodier stem base was found alongside decreases in the pith-to-xylem ratio, increases in the proportion of secondary xylem, and increases in lignin content. We show that there can be considerable variation in embolism resistance across the stem in herbs and that this variation is linked to the degree of secondary growth present. A gradient in embolism resistance across the stem in herbaceous plants could be an adaptation to ensure reproduction or basal resprouting during episodes of drought late in the lifecycle.

Cortical cell size regulates root metabolic cost

It has been hypothesized that vacuolar occupancy in mature root cortical parenchyma cells regulates root metabolic cost and thereby plant fitness under conditions of drought, suboptimal nutrient availability, and increased soil mechanical impedance. However, the mechanistic role of vacuoles in reducing root metabolic cost was unproven. Here we provide evidence to support this hypothesis. We first show that root cortical cell size is determined by both cortical cell diameter and cell length. Significant genotypic variation for both cortical cell diameter (~1.1- to 1.5-fold) and cortical cell length (~ 1.3- to 7-fold) was observed in maize and wheat. GWAS and QTL analyses indicate cortical cell diameter and length are heritable and under independent genetic control. We identify candidate genes for both phenes. Empirical results from isophenic lines contrasting for cortical cell diameter and length show that increased cell size, due to either diameter or length, is associated with reduced root respiration, nitrogen content, and phosphorus content. RootSlice, a functional-structural model of root anatomy, predicts that an increased vacuolar: cytoplasmic ratio per unit cortical volume causes reduced root respiration and nutrient content. Ultrastructural imaging of cortical parenchyma cells with varying cortical diameter and cortical cell length confirms the in silico predictions and shows that an increase in cell size is correlated with increased vacuolar volume and reduced cytoplasmic volume. Vacuolar occupancy and its relationship with cell size merits further investigation as a phene for improving crop adaptation to edaphic stress.

Unlocking dynamic root phenotypes for simultaneous enhancement of water and phosphorus uptake

Phosphorus (P) and water are crucial for plant growth, but their availability is challenged by climate change, leading to reduced crop production and global food security. In many agricultural soils, crop productivity is confronted by both water and P limitations. The diminished soil moisture decreases available P due to reduced P diffusion, and inadequate P availability diminishes tissue water status through modifications in stomatal conductance and a decrease in root hydraulic conductance. P and water display contrasting distributions in the soil, with P being concentrated in the topsoil and water in the subsoil. Plants adapt to water- and P-limited environments by efficiently exploring localized resource hotspots of P and water through the adaptation of their root system. Thus, developing cultivars with improved root architecture is crucial for accessing and utilizing P and water from arid and P-deficient soils. To meet this goal, breeding towards multiple advantageous root traits can lead to better cultivars for water- and P-limited environments. This review discusses the interplay of P and water availability and highlights specific root traits that enhance the exploration and exploitation of optimal resource-rich soil strata while reducing metabolic costs. We propose root ideotype models, including 'topsoil foraging', 'subsoil foraging', and 'topsoil/subsoil foraging' for maize (monocot) and common bean (dicot). These models integrate beneficial root traits and guide the development of water- and P-efficient cultivars for challenging environments.

Location: root architecture structures rhizosphere microbial associations

Root architectural phenotypes are promising targets for crop breeding, but root architectural effects on microbial associations in agricultural fields are not well understood. Architecture determines the location of microbial associations within root systems, which, when integrated with soil vertical gradients, determines the functions and the metabolic capability of rhizosphere microbial communities. We argue that variation in root architecture in crops has important implications for root exudation, microbial recruitment and function, and the decomposition and fate of root tissues and exudates. Recent research has shown that the root microbiome changes along root axes and among root classes, that root tips have a unique microbiome, and that root exudates change within the root system depending on soil physicochemical conditions. Although fresh exudates are produced in larger amounts in root tips, the rhizosphere of mature root segments also plays a role in influencing soil vertical gradients. We argue that more research is needed to understand specific root phenotypes that structure microbial associations and discuss candidate root phenotypes that may determine the location of microbial hotspots within root systems with relevance to agricultural systems.

Soil Water Deficit Reduced Root Hydraulic Conductivity of Common Reed (Phragmites australis)

Alterations in root hydraulics in response to varying moisture conditions remain a subject of debate. In our investigation, we subjected common reeds (Phragmites australis) to a 45-day treatment with four distinct soil moisture levels. The findings unveiled that, in response to drought stress, the total root length, surface area, volume, and average diameter exhibited varying degrees of reduction. Anatomically, drought caused a reduction in root diameter (RD), cortex thickness (CT), vessel diameter (VD), and root cross-sectional area (RCA). A decrease in soil moisture significantly reduced both whole- and single-root hydraulic conductivity (Lpwr, Lpsr). The total length, surface area, volume, and average diameter of the reed root system were significantly correlated with Lpwr, while RD, CT, and RCA were significantly correlated with Lpsr. A decrease in soil moisture content significantly influenced root morphological and anatomical characteristics, which, in turn, altered Lpr, and the transcriptome results suggest that this may be associated with the variation in the expression of abscisic acid (ABA) and aquaporins (AQPs) genes. Our initial findings address a gap in our understanding of reed hydraulics, offering fresh theoretical insights into how herbaceous plants respond to external stressors.

RootSlice-A novel functional-structural model for root anatomical phenotypes

Root anatomy is an important determinant of root metabolic costs, soil exploration, and soil resource capture. Root anatomy varies substantially within and among plant species. RootSlice is a multicellular functional-structural model of root anatomy developed to facilitate the analysis and understanding of root anatomical phenotypes. RootSlice can capture phenotypically accurate root anatomy in three dimensions of different root classes and developmental zones, of both monocotyledonous and dicotyledonous species. Several case studies are presented illustrating the capabilities of the model. For maize nodal roots, the model illustrated the role of vacuole expansion in cell elongation; and confirmed the individual and synergistic role of increasing root cortical aerenchyma and reducing the number of cortical cell files in reducing root metabolic costs. Integration of RootSlice for different root zones as the temporal properties of the nodal roots in the whole-plant and soil model OpenSimRoot/maize enabled the multiscale evaluation of root anatomical phenotypes, highlighting the role of aerenchyma formation in enhancing the utility of cortical cell files for improving plant performance over varying soil nitrogen supply. Such integrative in silico approaches present avenues for exploring the fitness landscape of root anatomical phenotypes.

Alfalfa in rotation with annual crops reduced nitrate leaching potential

Rotation of perennial alfalfa (Medicago sativa L.) with annual crops has the potential to reduce nitrate-nitrogen (NO₃ -N) in the vadose zone and increase soil organic carbon (SOC) sequestration. The objective of this study was to determine the long-term effects on SOC, NO₃ -N, ammonium-N (NH₄ -N), and soil water in the 7.2 m depth with an alfalfa rotation compared with continuous corn (Zea mays L.). Soils from six pairs of alfalfa rotation versus continuous corn observation points were sampled to 7.2 m depth in 0.3 m increments. The uppermost 0.3 m was divided into 0-0.15 and 0.15-0.30 m. For the 0-7.2 m depth, the alfalfa rotation compared with continuous corn had 26% less soil water (0.29 vs. 0.39 g cm^-3 ) and 55% less NO₃ -N (368 vs. 824 kg ha^-1 ). The cropping system and NO₃ -N concentration did not affect NH₄ -N in the vadose zone. The alfalfa rotation compared with continuous corn had 47% higher SOC (105.96 Mg ha^-1  vs. 72.12 Mg ha^-1 ) and 23% higher total soil nitrogen (TSN) (11.99 Mg ha^-1  vs. 9.73 Mg ha^-1 ) in the 0-1.2 m depth. The greater depletion of soil water and NO₃ -N with alfalfa rotation was primarily below the rooting zone of corn, suggesting no negative implications for corn following alfalfa but greatly reduced potential of NO₃ -N leaching to the aquifer with the alfalfa rotation. Alfalfa rotation compared with continuous corn is a means to greatly reduce the leaching of NO₃ -N to the aquifer and improve the surface soil with the potential to increase SOC sequestration.

Drought- and soil substrate-induced variations in root nonstructural carbohydrates result from fine root morphological and anatomical traits of Juglans mandshurica seedlings

BACKGROUND

Nonstructural carbohydrates (NSCs) reflect the carbon supply status and affect the construction and development of plants. Previous studies have focused on the dynamics of NSCs among plant organs, however, few studies have paid attention to the synergistic variations between fine root traits and NSCs under drought based on the perspective of branch order roots. This study aims to explore the responses of fine root traits and NSCs among root orders of Juglans mandshurica seedlings under different drought intensities and soil substrates. The 2-year-old J. mandshurica potted seedlings were planted in three different soil substrates (humus, loam and sandy-loam soil) and subjected to four drought intensities (CK, mild drought T1, moderate drought T2 and severe drought T3) for 60 days.

RESULTS

The root biomass of seedlings in sandy-loam soil under the same drought intensity was higher than that of seedlings in humus soil. With an increase in drought, the root biomass, average diameter, root tissue density and cortex thickness decreased significantly, and the specific root length, stele diameter and conduit density increased. The root NSC contents in humus soil were higher than those in sandy-loam soil. The fine root soluble sugar content in all soil substrates decreased with increasing drought intensity, while the root starch and total NSC contents varied among the different soil substrates. Compared with transportive roots, the morphological and anatomical traits jointly explained the higher variation in NSC contents of the absorptive roots. The anatomical traits explained the higher variation in the NSC content of first five order roots.

CONCLUSION

Our results suggest that coordinated adaptation of the root traits and NSCs of Manchurian walnut seedlings exposed to water gradients in different soil substrates.

Root and xylem anatomy varies with root length, root order, soil depth and environment in intermediate wheatgrass (Kernza®) and alfalfa

BACKGROUND AND AIMS

Deep roots (i.e. >1 m depth) are important for crops to access water when the topsoil is dry. Root anatomy and hydraulic conductance play important roles in the uptake of soil water, particularly water located deep in the soil. We investigated whether root and xylem anatomy vary as a function of root type, order and length, or with soil depth in roots of two deep-rooted perennial crops: intermediate wheatgrass [Thinopyrum intermedium (Kernza®)] and alfalfa (Medicago sativa). We linked the expression of these anatomical traits to the plants' capacity to take up water from deep soil layers.

METHODS

Using laser ablation tomography, we compared the roots of the two crops for cortical area, number and size of metaxylem vessels, and their estimated root axial hydraulic conductance (ERAHCe). The deepest roots investigated were located at soil depths of 2.25 and at 3.5 m in the field and in rhizoboxes, respectively. Anatomical differences were characterized along 1-m-long individual roots, among root types and orders, as well as between environmental conditions.

KEY RESULTS

For both crops, a decrease in the number and diameter, or both, of metaxylem vessels along individual root segments and with soil depth in the field resulted in a decrease in ERAHCe. Alfalfa, with a greater number of metaxylem vessels per root throughout the soil profile and, on average, a 4-fold greater ERAHCe, took up more water from the deep soil layers than intermediate wheatgrass. Root anatomical traits were significantly different across root types, classes and growth conditions.

CONCLUSIONS

Root anatomical traits are important tools for the selection of crops with enhanced exploitation of deep soil water. The development and breeding of perennial crops for improved subsoil exploitation will be aided by greater understanding of root phenotypes linked to deep root growth and activity.

Characterization, costs, cues and future perspectives of phenotypic plasticity

BACKGROUND

Plastic responses of plants to the environment are ubiquitous. Phenotypic plasticity occurs in many forms and at many biological scales, and its adaptive value depends on the specific environment and interactions with other plant traits and organisms. Even though plasticity is the norm rather than the exception, its complex nature has been a challenge in characterizing the expression of plasticity, its adaptive value for fitness and the environmental cues that regulate its expression.

SCOPE

This review discusses the characterization and costs of plasticity and approaches, considerations, and promising research directions in studying plasticity. Phenotypic plasticity is genetically controlled and heritable; however, little is known about how organisms perceive, interpret and respond to environmental cues, and the genes and pathways associated with plasticity. Not every genotype is plastic for every trait, and plasticity is not infinite, suggesting trade-offs, costs and limits to expression of plasticity. The timing, specificity and duration of plasticity are critical to their adaptive value for plant fitness.

CONCLUSIONS

There are many research opportunities to advance our understanding of plant phenotypic plasticity. New methodology and technological breakthroughs enable the study of phenotypic responses across biological scales and in multiple environments. Understanding the mechanisms of plasticity and how the expression of specific phenotypes influences fitness in many environmental ranges would benefit many areas of plant science ranging from basic research to applied breeding for crop improvement.

Using anatomical traits to understand root functions across root orders of herbaceous species in a temperate steppe

Root anatomical traits play crucial roles in understanding root functions and root form-function linkages. However, the root anatomy and form-function linkages of monocotyledonous and dicotyledonous herbs remain largely unknown. We measured order-based anatomical traits and mycorrhizal colonization rates of 32 perennial herbs of monocotyledons and dicotyledons in a temperate steppe. For monocots, relative constant proportion of cortex and mycorrhizal colonization rates, but increased cell-wall thickening of the endodermis and proportion of stele were observed across root orders, indicating a slight reduction in absorption capacity and improvement in transportation capacity across orders. For dicots, the cortex and mycorrhizal colonization disappeared in the fourth-order and/or fifth-order roots, whereas the secondary vascular tissue increased markedly, suggesting significant transition of root functions from absorption to transportation across root orders. The allometric relationships between stele and cortex differed across root orders and plant groups, suggesting different strategies to coordinate the absorption and transportation functions among plant groups. In summary, our results revealed different functional transition patterns across root orders and distinct strategies for coordinating the absorption and transportation of root system between monocots and dicots. These findings will contribute to our understanding of the root form and functions in herbaceous species.

Root pressure-volume curve traits capture rootstock drought tolerance

BACKGROUND AND AIMS

Living root tissues significantly constrain plant water uptake under drought, but we lack functional traits to feasibly screen diverse plants for variation in the drought responses of these tissues. Water stress causes roots to lose volume and turgor, which are crucial to root structure, hydraulics and growth. Thus, we hypothesized that root pressure-volume (p-v) curve traits, which quantify the effects of water potential on bulk root turgor and volume, would capture differences in rootstock drought tolerance.

METHODS

We used a greenhouse experiment to evaluate relationships between root p-v curve traits and gas exchange, whole-plant hydraulic conductance and biomass under drought for eight grapevine rootstocks that varied widely in drought performance in field trials (101-14, 110R, 420A, 5C, 140-Ru, 1103P, Ramsey and Riparia Gloire), grafted to the same scion variety (Vitis vinifera 'Chardonnay').

KEY RESULTS

The traits varied significantly across rootstocks, and droughted vines significantly reduced root turgor loss point (πtlp), osmotic potential at full hydration (πo) and capacitance (C), indicating that roots became less susceptible to turgor loss and volumetric shrinkage. Rootstocks that retained a greater root volume (i.e. a lower C) also maintained more gas exchange under drought. The rootstocks that previous field trials have classified as drought tolerant exhibited significantly lower πtlp, πo and C values in well-watered conditions, but significantly higher πo and πtlp values under water stress, than the varieties classified as drought sensitive.

CONCLUSIONS

These findings suggest that acclimation in root p-v curve traits improves gas exchange in persistently dry conditions, potentially through impacts on root hydraulics or root to shoot chemical signalling. However, retaining turgor and volume in previously unstressed roots, as these roots deplete wet soil to moderately negative water potentials, could be more important to drought performance in the deep, highly heterogenous rooting zones which grapevines develop under field conditions.

Improving Soil Resource Uptake by Plants Through Capitalizing on Synergies Between Root Architecture and Anatomy and Root-Associated Microorganisms

Root architectural and anatomical phenotypes are highly diverse. Specific root phenotypes can be associated with better plant growth under low nutrient and water availability. Therefore, root ideotypes have been proposed as breeding targets for more stress-resilient and resource-efficient crops. For example, root phenotypes that correspond to the Topsoil Foraging ideotype are associated with better plant growth under suboptimal phosphorus availability, and root phenotypes that correspond to the Steep, Cheap and Deep ideotype are linked to better performance under suboptimal availability of nitrogen and water. We propose that natural variation in root phenotypes translates into a diversity of different niches for microbial associations in the rhizosphere, rhizoplane and root cortex, and that microbial traits could have synergistic effects with the beneficial effect of specific root phenotypes. Oxygen and water content, carbon rhizodeposition, nutrient availability, and root surface area are all factors that are modified by root anatomy and architecture and determine the structure and function of the associated microbial communities. Recent research results indicate that root characteristics that may modify microbial communities associated with maize include aerenchyma, rooting angle, root hairs, and lateral root branching density. Therefore, the selection of root phenotypes linked to better plant growth under specific edaphic conditions should be accompanied by investigating and selecting microbial partners better adapted to each set of conditions created by the corresponding root phenotype. Microbial traits such as nitrogen transformation, phosphorus solubilization, and water retention could have synergistic effects when correctly matched with promising plant root ideotypes for improved nutrient and water capture. We propose that elucidation of the interactive effects of root phenotypes and microbial functions on plant nutrient and water uptake offers new opportunities to increase crop yields and agroecosystem sustainability.

Future roots for future soils

Mechanical impedance constrains root growth in most soils. Crop cultivation changed the impedance characteristics of native soils, through topsoil erosion, loss of organic matter, disruption of soil structure and loss of biopores. Increasing adoption of Conservation Agriculture in high-input agroecosystems is returning cultivated soils to the soil impedance characteristics of native soils, but in the low-input agroecosystems characteristic of developing nations, ongoing soil degradation is generating more challenging environments for root growth. We propose that root phenotypes have evolved to adapt to the altered impedance characteristics of cultivated soil during crop domestication. The diverging trajectories of soils under Conservation Agriculture and low-input agroecosystems have implications for strategies to develop crops to meet global needs under climate change. We present several root ideotypes as breeding targets under the impedance regimes of both high-input and low-input agroecosystems, as well as a set of root phenotypes that should be useful in both scenarios. We argue that a 'whole plant in whole soil' perspective will be useful in guiding the development of future crops for future soils.

Biogeochemical cycles of nutrients, rare earth elements (REEs) and Al in soil-plant system in ion-adsorption REE mine tailings remediated with amendment and ramie (Boehmeria nivea L.)

The exploitation of ion-adsorption rare earth element (REE) deposits in South China has left large areas of mine tailings. However, limited remediation practices on these tailings have been reported, and how the remediation strategies and economic plants cultivation affect the biogeochemical cycles of nutrients, REEs and Al remains unclear. The aim of the present study was to investigate the effects of the combination of the addition of soil amendment and the root development and activity of a fiber plant ramie (Boehmeria nivea L.) on the availability and distribution of nutrients, as well as of REEs and other potentially toxic elements (e.g. Al) in the soil-plant system. The results showed that the application of organic amendment and ramie planting induced a significant increase in soil pH, total carbon (C), nitrogen (N), and other nutrient (e.g. P and Ca) concentrations, while led to a decrease of 80-90% and 60-90% in soil extractable REE and Al concentrations respectively. Matrices of correlation showed that soil pH, total C, N, and P concentrations were among the most important factors controlling the availability of soil REEs and Al, and root characteristics (e.g. fine root length). The total C, N, P and extractable nutrient concentrations, and electrical conductivity were higher in the rhizosphere soils of ramie than those in the bulk soils. Moreover, more than 60% of the quantity of REE and Al in the whole ramie plant was stored within the thick roots. These results showed that, in addition to amendment, the effects induced by the roots of ramie could further improve soil properties through C input, nutrient mobilization and toxic element stabilization. Our study concludes that ramie planting with organic amendment is a promising phytostabilization strategy for the remediation of REE mine tailings in South China.

Harnessing root architecture to address global challenges

Root architecture can be targeted in breeding programs to develop crops with better capture of water and nutrients. In rich nations, such crops would reduce production costs and environmental pollution and, in developing nations, they would improve food security and economic development. Crops with deeper roots would have better climate resilience while also sequestering atmospheric CO₂ . Deeper rooting, which improves water and N capture, is facilitated by steeper root growth angles, fewer axial roots, reduced lateral branching, and anatomical phenotypes that reduce the metabolic cost of root tissue. Mechanical impedance, hypoxia, and Al toxicity are constraints to subsoil exploration. To improve topsoil foraging for P, K, and other shallow resources, shallower root growth angles, more axial roots, and greater lateral branching are beneficial, as are metabolically cheap roots. In high-input systems, parsimonious root phenotypes that focus on water capture may be advantageous. The growing prevalence of Conservation Agriculture is shifting the mechanical impedance characteristics of cultivated soils in ways that may favor plastic root phenotypes capable of exploiting low resistance pathways to the subsoil. Root ideotypes for many low-input systems would not be optimized for any one function, but would be resilient against an array of biotic and abiotic challenges. Root hairs, reduced metabolic cost, and developmental regulation of plasticity may be useful in all environments. The fitness landscape of integrated root phenotypes is large and complex, and hence will benefit from in silico tools. Understanding and harnessing root architecture for crop improvement is a transdisciplinary opportunity to address global challenges.

Many paths to one goal: Identifying integrated rice root phenotypes for diverse drought environments

Drought is a major source of yield loss in the production of rice (Oryza sativa L.), and cultivars that maintain yield under drought across environments and drought stress scenarios are urgently needed. Root phenotypes directly affect water interception and uptake, so plants with root systems optimized for water uptake under drought would likely exhibit reduced yield loss. Deeper nodal roots that have a low metabolic cost per length (i.e., cheaper roots) via smaller root diameter and/or more aerenchyma and that transport water efficiently through smaller diameter metaxylem vessels may be beneficial during drought. Subsets of the Rice Diversity Panel 1 and Azucena × IR64 recombinant inbred lines were grown in two greenhouse and two rainout shelter experiments under drought stress to assess their shoot, root anatomical, and root architectural phenotypes. Root traits and root trait plasticity in response to drought varied with genotype and environment. The best-performing groups in the rainout shelter experiments had less plasticity of living tissue area in nodal roots than the worst performing groups. Root traits under drought were partitioned into similar groups or clusters via the partitioning-around-medoids algorithm, and this revealed two favorable integrated root phenotypes common within and across environments. One favorable integrated phenotype exhibited many, deep nodal roots with larger root cross-sectional area and more aerenchyma, while the other favorable phenotype exhibited many, deep nodal roots with small root cross-sectional area and small metaxylem vessels. Deeper roots with high theoretical axial hydraulic conductance combined with reduced root metabolic cost contributed to greater shoot biomass under drought. These results reflect how some root anatomical and architectural phenes work in concert as integrated phenotypes to influence the performance of plant under drought stress. Multiple integrated root phenotypes are therefore recommended to be selected in breeding programs for improving rice yield across diverse environments and drought scenarios.

Impact of water deficit on the development and senescence of tomato roots grown under various soil textures of Shaanxi, China

PURPOSE

Water scarcity is expected to extend to more regions of the world and represents an alarming threat to food security worldwide. Under such circumstances, water holding capacity is an important agronomic trait, which is primarily controlled by soil texture.

METHODS

Our work examined three different soil textures from three cities of Shaanxi Province in China, i.e., silt-sandy loam from Yulin (north of Shaanxi), loam-clay loam from Yangling (middle and western part of Shaanxi), and clay loam-clay from Hanzhong soil (south of Shaanxi), at two moisture levels, i.e., field capacity of 70-75% (well-watered) and 50-55% (water deficit).

RESULTS

The differences in soil particle sizes altered the soil physiochemical properties and soil enzymatic activities. Soil urease and ß-glucosidase activities were significantly higher in the Yangling soil under the well-watered treatment, while the differences were nonsignificant under the water deficit conditions. The leaf photosynthesis rate and total chlorophyll content were significantly higher in Hanzhong soil after 15 days of treatment; however, the overall highest plant length, root cortex diameter, and xylem element abundance were significantly higher in Yangling soil under the water deficit conditions. Furthermore, comparable differences were observed in antioxidant defence enzymes and endogenous hormones after every 15 days of treatments. The auxin, gibberellic acid and cytokinin concentrations in leaves and roots were comparably high in Yangling soil, while the abscisic acid concentrations were higher in Hanzhong soil under the water deficit conditions.

CONCLUSIONS

Our findings concluded that soil compaction has a significant role not only in root morphology, growth, and development but also in the soil physicochemical properties and nutrient cycle, which are useful for the growth and development of tomato plants.

Root morphophysiology changes during the habitat transition from soil to canopy of the aroid vine Rhodospatha oblongata

BACKGROUND AND AIMS

The aroid vine Rhodospatha oblongata is characterized by a habitat change from terrestrial to canopy, relying on aerial roots at maturity to obtain water and nutrients from the forest soil. We hypothesize that morphophysiological acclimation occurs in roots as they grow under atmospheric conditions. These changes would guarantee the whole-plant survival of aroid vines in the new and potentially stressful habitat of the canopy.

METHODS

Terrestrial and aerial roots were compared on a morphophysiological basis. Root anatomy, water balance, water absorption capacity via fluorescent tracer, and photochemical activity via chlorophyll fluorescence were measured.

KEY RESULTS

While thin fasciculate roots occur on terrestrial crawling individuals, two clearly distinct aerial roots (anchor and feeder) are produced on canopy individuals, which both adhere to the host trunk. The colour of both aerial roots changes during development from red and brownish to striped and green at maturity. Colour changes are induced by the replacement of epidermis, exodermis and outer cortex by an inner layer of lignified cork on the root region exposed to the atmosphere. In the root region that is in contact with the host, covering substitutions do not occur and both exodermis and lignified cork, along with several epidermal hairs, appear. Water retention capacity was higher in green roots than in other root types. Rehydration capacity via water absorption by hairs of aerial roots was confirmed by fluorescence. Chlorophyll fluorescence data indicated low levels of photosynthetic capacity in aerial roots.

CONCLUSIONS

Plants should evolve strategies to survive stress situations. The transition from soil to canopy imposes abiotic changes and potentially stressful situations on R. oblongata. We conclude that the morphophysiological changes observed represent an important strategy that permits the maintenance of aroid roots and the survival of R. oblongata in the canopy.

Root metaxylem and architecture phenotypes integrate to regulate water use under drought stress

At the genus and species level, variation in root anatomy and architecture may interact to affect strategies of drought avoidance. To investigate this idea, root anatomy and architecture of the drought-sensitive common bean (Phaseolus vulgaris) and drought-adapted tepary bean (Phaseolus acutifolius) were analyzed in relation to water use under terminal drought. Intraspecific variation for metaxylem anatomy and axial conductance was found in the roots of both species. Genotypes with high-conductance root metaxylem phenotypes acquired and transpired more water per unit leaf area, shoot mass, and root mass than genotypes with low-conductance metaxylem phenotypes. Interspecific variation in root architecture and root depth was observed where P. acutifolius has a deeper distribution of root length than P. vulgaris. In the deeper-rooted P. acutifolius, genotypes with high root conductance were better able to exploit deep soil water than genotypes with low root axial conductance. Contrastingly, in the shallower-rooted P. vulgaris, genotypes with low root axial conductance had improved water status through conservation of soil moisture for sustained water capture later in the season. These results indicate that metaxylem morphology interacts with root system depth to determine a strategy of drought avoidance and illustrate synergism among architectural and anatomical phenotypes for root function.